The present invention relates to aluminum brazing alloys for assembling aluminum heat exchangers, especially suited for assembling plate fin heat exchangers adapted for superhigh pressure service by brazing.
Plate fin heat exchangers made of aluminum have been heretofore fabricated by means of an appropriate brazing technique such as vacuum brazing , atmospheric brazing or dip brazing, wherein aluminum brazing alloys containing 4.5 to 13.5% Si and optionally, the addition of less than 3% Mg, or 2.3 to 4.7% Cu and 9.3 to 10.7% Zn have been commonly employed. The above-mentioned silicon containing brazing alloys may further contain Be and Bi. Throughout the present specification, percentages are by weight unless otherwise indicated.
Superhigh pressure service plate fin heat exchanger is specified by the rupture pressure when the heat exchanger is ruptured due to the internal pressure. Strength at brazed joint portions is regarded as a dominant factor of the rupture pressure and the strength mainly depends on the width of the fillet and the structure in the brazed joint portions. However, in the fabrication of large scale heat exchangers, a prolonged brazing time does not permit a sufficiently wide fillet and, thus, the currently used heat exchangers exhibit a low rupture pressure.
On the other hand, in general, the metallurgical structure of the brazed portion can be refined by increasing the cooling rate after brazing, whereby the strength can be increased. However, practically, it is impossible to increase adequately the cooling rate in the construction of the large-sized heat exchangers and, thus, refinement of the structure cannot be achieved. For the foregoing reasons, improvement in rupture pressure can not be expected in the prior art.